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Experiments were performed to obtain information on: (i) the specific properties of Ca2+ binding and transport in yeast; (ii) the relationship between both parameters; (iii) similarities to or differences from other biological systems as measured by the effects of inhibitors; and (iv) the effects of mono and divalent cations, in order to get some insight on the specificity and some characteristics of the mechanism of the transport system for divalent cations in yeast. The results obtained gave some kinetic parameters for a high affinity system involved in the transport of Ca2+ in yeast. These were obtained mainly by considering actual concentrations of Ca2+ in the medium after substracting the amounts bound to the cell. A km of 1.9 microM and a Vmax of 1.2 nmol (100 mg.3 min)-1 were calculated. The effects of some inhibitors and other cations on Ca2+ uptake allow one to postulate some independence between binding and transport for this divalent cation. Of the inhibitors tested, only lanthanum seems to be a potent inhibitor of Ca2+ uptake in yeast. The effects of Mg2+ on the uptake of Ca2+ agree with the existence of a single transport system for both divalent cations. The actions of Na+ and K+ on the transport of Ca2+ offer interesting possibilities to study further some of the mechanistic properties of this transport system for divalent cations.

Synchronous culture of the budding yeast Saccharomyces cerevisiae was obtained by sucrose density gradient selection with 90-100% of yeast synchronized by using the cells in the bottom. In these adult cells bud emergence is coincident with an increase in calcium uptake at 1OOmin of the culture, followed by a return to basal values which are maintained until the end of the first cell cycle of study. The phenothiazine derivatives, trifluoperazine and chlorpromazine inhibit bud emergence and trifluoperazine also increases calcium uptake.

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